Coax switches – is shorting unused port necessary for isolation?

At Ratings of coax antenna switches I showed characteristics of a home made switch which has very low InsertionVSWR, but poor isolation.

A couple of correspondents have offered an explanation that the unused port must be shorted to get good isolation.

If that was the case, then we would expect all coax switches that leave the unused port open to have poor isolation.

Let us look at a very good coaxial relay


Above is a Dowkey 402 series relay which has good performance to GHz. It does not short the unused port.
Continue reading Coax switches – is shorting unused port necessary for isolation?

Ratings of coax antenna switches

In a recent article I discussed how InsertionLoss implies InsertionVSWR in lossless devices.

This article looks at measurements of a few antenna switches at hand.

Daiwa CS-201G II

It is difficult to find comprehensive data on the very popular Daiwa CS-201 series switches.


Above is the data from the packet of one of these switches, a CS-201G II. The specifications are pretty loose, and one must depend on one’s own measurements.


Above, the CS-201G II, a basic CS-201 series switch with N connectors, advertised as useful to 2000MHz where InsertionLoss is given as 1.2dB (or better?). If there were no TransmissionLoss in the switch, that would imply InsertionVSWR=3.6, but there is probably some significant TransmissionLoss and InsertionVSWR would be somewhat less. Nevertheless, IMHO InsertionLoss=1.2dB indicates it as unsuitable such frequencies. Continue reading Ratings of coax antenna switches

InsertionLoss implies InsertionVSWR in lossless devices

Devices inserted in transmission lines often characterised by one or more of:

  1. Insertion VSWR (the input VSWR when terminated with a matched load);
  2. Return Loss (RL) in dB (20 times the log of the magnitude of the complex reflection coefficient); and
  3. Insertion Loss.

Practitioners often find Insertion VSWR (1) of most use as it indicates whether the device is worse than other system devices, the weak link in the chain if you like. You might see a coax antenna switch specified to have InsertionVSWR<1.2 to 60MHz.

Return Loss (2) is a function of VSWR and vice versa, so it appeals when the designer thinks in terms of Return Loss rather than VSWR (and it is a better metric for VSWR<1.2). You might see a coaxial relay specified to have ReturnLoss>30dB to 500MHz.

Insertion Loss (3) is not so readily compared to the other two which are measures of input reflection with a matched termination. It often yields some numbers that appear very acceptable, but might be deceptively so. You might see a coaxial relay specified to have ReturnLoss>30dB to 500MHz. You might see a coax antenna switch specified to have InsertionLoss<0.2dB to 100MHz. Continue reading InsertionLoss implies InsertionVSWR in lossless devices

Seriously Pro F3 flight controller gyro/acc failure

I purchased an Acro Seriously Pro F3 flight controller (FC) recently and having soldered the connectors on and loaded the current firmware, it was time for a pre-installation checkout.

The article outlines a basic pre-installation test that revealed problems that would prevent effective use of the FC.

Acc/gyro checkout


Doing no more than an acc calibration and allowing initialisation with a stabilised FC, the FC on the end of a USB cable should deliver acceptable sensor responses in Cleanflight Configurator (CC).

This flight controller has two problems:

  • offset in the gyro z axis (yaw);
  • low acc output in the Y axis (roll).

Continue reading Seriously Pro F3 flight controller gyro/acc failure

Flashing SeriouslyProF3 Cleanflight using ST-LINKV2


If for some reason you cannot use the bootloader that reads from the USB port via the CP2102, you can resort to programming the MCU chip using the SWD port and a programmer that supports that protocol.

WARNING: there is potential for damage if you get this stuff wrong. No warranty is offered, if you break it, you get to keep both parts.

Example hardware configuration


Above is the hardware configuration for programming. I have used a LiPo and BEC to supply 5V to the SPF3 board (the target), and a clone ST-LINKV2 (~$4 on eBay) is connected to the Serial Wire Debug (SWD) connector on the SPF3 with a custom cable. Note that you cannot use USART2 concurrently with SWD.
Continue reading Flashing SeriouslyProF3 Cleanflight using ST-LINKV2

Cleanflight – initialisation failure beeps

I spent quite a while chasing down a problem in a Cleanflight v1.13 flight controller which would not start properly, giving a series of beeps that one might expect hinted the problem.


Above is a scope capture of the /BEEP signal, five short beeps (50ms ON, 50ms OFF) then four long beeps (250ms ON, 250ms OFF). Continue reading Cleanflight – initialisation failure beeps

A drive test of RTKLIB accurate GPS

Accurate GPS positioning has been of interest in automating data collection in field strength surveys of antennas on HF. To be useful, the positions need to be accurate to 100mm.

RTKLIB is an open source program package for GNSS positioning.

A number of experiments were conducted using a U-Blox LEA-6T assembly with integral small patch antenna (cost ~A$40) and RTKLIB v2.4.3(b8) using a base station feed from Geoscience Australia (with thanks). The test location is about 200km from the reference station at Symonston ACT.


Above, the LEA-6T and a RS-232 to USB adapter are strapped to a roof rack bar with stretch film. The high mounting position gives good view of the sky.


Screenshot - 30_06_16 , 12_50_35

Above is a position plot of about an hour with the vehicle stationary (though in Kinematic mode) beside the house. Sky view was slightly restricted by the house and a line of tall trees to the west.

firefox 30/06/16 , 15:36:58 20160630 - Mozilla Firefox

For the most part the track is smooth, but there is one section where the track in a more heavily treed section is markedly jaggy.

Atten APS3005S – a better thermostatic fan control

The Atten APS3005S is a 0-30V 5A linear DC power supply.


This later model includes a thermostatically controlled fan which at moderately light currents short cycles (10s on 20s off) and is very annoying… especially since it sits above my desk.

This project describes application of the generic heating / cooling controller (hcctl) to control the fan, reducing the short cycling nuisance.
Continue reading Atten APS3005S – a better thermostatic fan control

Exploiting your antenna analyser #22

On a transmission line with standing waves, the voltage varies cyclically along the line, and is dependent also on power.

This article explains a method to use an analyser to predict the peak voltage level at a point for a given frequency and power based on measurement or estimation of complex Z or Y at that point using a suitable antenna analyser.

The problem

Lets say you have some critical  voltage breakdown limit and want to use your analyser to find any non-compliance at the proposed power level.

Let us assume that the not-to-exceed voltage at that point is 1000Vpk. Let’s allow a little margin for variation due to factors not fixed, let’s actually use 800Vpk as the limit. We will use the maximum permitted power in Australia, 400W.
Continue reading Exploiting your antenna analyser #22

Silver vs alkaline button cells

I purchased a new digital caliper recently (no, they are NOT vernier calipers, though modern usage seems to have misused the term vernier to the point of it having no value).


A pic of the back reveals their recommendation for a battery, it is in the upper right corner of the pic “Battery 1.55V”. This is really subtle and a departure from previous practice of marking them more clearly SR44.

The nominal voltage of a silver button cell is 1.55V, an alkaline is 1.5V. Continue reading Silver vs alkaline button cells